Analysis Of Rolling Temperature On The Microstructure And Properties Of Titanium Forgings

The quality of titanium forgings directly determines their service reliability. As a core process parameter in the hot working process, rolling temperature profoundly affects the final mechanical properties of products by regulating the phase transformation behavior and microstructural evolution inside the forgings.

 

 

The crystal structure of titanium alloys changes with temperature, with an α/β phase transformation critical point (885~900℃ for commercially pure titanium and 980~1010℃ for Gr5 alloy). Based on this, rolling is divided into three categories: β phase zone, α+β phase zone, and critical zone, with significant differences in microstructural evolution and properties.

 

1. β Phase Zone Rolling (> Phase Transformation Point)

The billet consists of body-centered cubic (BCC) β phase, featuring low deformation resistance and good plasticity, making it suitable for large-deformation processing. Ingots can undergo 70%~80% large deformation to break coarse grains and form a uniform fibrous structure. However, acicular martensite is prone to form after cooling, leading to an imbalance between strength, plasticity, and toughness, which requires subsequent heat treatment optimization.

 

2. α+β Phase Zone Rolling (< Phase Transformation Point)

This is the core rolling interval for finished forgings, mostly controlled at 30~50℃ below the phase transformation point (e.g., 950~800℃ for Gr5 die forging). The material consists of a dual phase of hexagonal close-packed (HCP) α phase and BCC β phase. Deformation is accompanied by grain fragmentation and α phase refinement/spheroidization, enabling the formation of an ideal structure of equiaxed α phase + lamellar α-transformed β phase, which balances strength and plasticity.

 

3. Critical Zone Rolling (Near Phase Transformation Point)

The structure is mixed and uneven, resulting in prone-to-fluctuate forging properties. It is not recommended without special requirements.

 

 

 

 

1. β Phase Zone Rolling

Acicular martensite forms after cooling, resulting in high strength but low plasticity and toughness. Insufficient deformation tends to retain original β grain boundaries and generate continuous grain boundary α phase, reducing toughness, causing stress concentration, and affecting service safety.

 

2. α+β Phase Zone Rolling

This is the optimal choice for balancing strength and plasticity. Reasonable temperature control can refine grains and optimize phase composition to improve properties.

 

3. Property Uniformity

Large-sized forgings are prone to differences in surface-core structure/properties due to temperature gradients. Optimizing temperature systems (e.g., multi-pass rolling) can improve this.

 

 

1. α-Type and Near-α-Type Titanium Alloys

Ingot breakdown requires a relatively high β phase zone temperature (1180~900℃) to reduce deformation resistance and improve productivity. Preforming and die forging need to be reduced to the α+β phase zone to ensure good microstructural properties. These alloys are highly sensitive to rolling temperature; excessively high temperatures easily lead to grain growth, while excessively low temperatures increase deformation resistance and prone to cracking.

 

2. α+β-Type Titanium Alloys (e.g., Gr5)

As the most widely used type, they have a wide rolling temperature range, but finished forgings must be strictly controlled in the α+β phase zone. Taking Gr5 as an example, the ingot breakdown temperature is 1200~850℃ (β phase zone), the preforming temperature is 1000~800℃ (α+β phase zone near the phase transformation point), and the die forging temperature is 950~800℃ (typical α+β phase zone). Through multi-stage temperature control, both processing efficiency and product performance are balanced.

 

3. Near-β-Type Titanium Alloys

These alloys have a low phase transformation temperature and can be rolled over a wide temperature range, but excessively high temperatures that cause excessive β grain growth must be avoided. They are usually rolled in the α+β phase zone to obtain a structure balancing strength and toughness.

 

 

1. Accurate Localization of Phase Transformation Temperature

Determine the α/β phase transformation point of specific alloys through thermal expansion experiments or metallographic analysis to divide the β phase zone and α+β phase zone, avoiding microstructural defects caused by misjudgment of intervals.

 

2. Select Rolling Intervals as Needed

Prioritize large-deformation rolling in the β phase zone for ingot breakdown to improve the original structure, and prioritize the α+β phase zone for finished forgings to balance strength and plasticity. For forgings requiring high toughness, the rolling temperature in the α+β phase zone can be appropriately reduced to refine grains.

 

3. Optimize Temperature Gradient Control

Adopt "low-temperature rapid rolling" or "multi-pass rolling" for large-sized forgings to reduce the surface-core temperature difference and improve property uniformity.

 

4. Synergize with Subsequent Heat Treatment

Anneal after β phase zone rolling to refine the acicular structure, and perform solution aging after α+β phase zone rolling to further improve strength.

 

Ruihang is a professional manufacturer of titanium and titanium alloy products, supplying high-quality titanium alloy forgings. For more details,please contact us via the Email: Sam.Rui@bjrh-titanium.com